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How to Determine Diagonal Mirror Size for a Telescope

Newtonian telescopes use a mirror placed at the lower end of an open tube to reflect and focus light back up the tube. A small circular mirror located in the upper part of the tube intercepts the reflected light and redirects it through the side of tube where the eyepiece is located. This mirror is situated in the center of the light path and blocks a small amount of incoming light before it is reflected back up the tube. Selecting a diagonal mirror that is large enough to capture all available light, without being so large as to obstruct incoming light, is critical for optimal telescope performance.

Things You'll Need

Ruler
Calculator

Instructions

- 1
  Find the focal ratio of the primary mirror. This is a number found on the telescope data plate or within the instruction booklet and will look something like this: f/4, f/8 or f/11 (the actual number for a particular telescope may vary).
- 2
  Calculate the focal length of the telescope by multiplying the diameter of the primary mirror by the focal ratio. For example, a telescope with an 8-inch primary mirror and a focal ratio of f/6 would have a focal length of 48 inches (8 x 6).
- 3
  Insert a small length of wood or a long ruler down the tube of the telescope and allow it to make gentle contact with the primary mirror. Look through the side tube, without an eyepiece in place, and mark the length of wood or take note of the measurement on the ruler (you are measuring the distance from the primary mirror to the diagonal mirror position).
- 4
  Subtract the distance measured from the telescope's primary mirror to the diagonal mirror position from the focal length. For example, if the focal length of the telescope is 48 inches and the distance from the primary mirror to the diagonal mirror position is 39 inches, then the distance from the diagonal mirror to the focal point is 9 inches (48 - 39).
- 5
  Divide the distance from the diagonal mirror position to the focal point by the the focal ratio of the telescope to determine the minimum diameter of the diagonal mirror. For example, if we use the figures calculated earlier, 9 / 6 = 1.5 inches.

Astronomy